The present invention relates to a radio frequency (RF) circuit having a function capable of amplifying a signal in an RF range, such as UHF waves, microwaves or milliwaves or converting the frequency of the same.
FIG. 15 is a block diagram showing a conventional RF circuit disclosed in, for example, The Institute of Electronics, Information and Communication Engineers (IEICE) Technical Report, MW 96-72, pp. 27-33, 1996, and composed of multi-stage amplifiers and a mixer. Referring to the drawing, reference numeral 1 represents an input terminal (IN), 2 represents an output terminal (OUT), 3 represents an input terminal (RF IN) for receiving an RF signal, 4 represents an input terminal (LO IN) for receiving a local signal, 5 represents an output terminal (IF OUT) for outputting an intermediate-frequency signal, 14a and 14b represent BJT amplifiers each of which is composed of a silicon bipolar junction transistor (hereinafter called as a "BJT"), 15 represents a mixer having the silicon BJT and 20 represents a silicon semiconductor chip on which two BJT amplifiers 14a and 14b and the mixer 15 are integrated.
The operation will now be described. The RF signal received by an antenna or the like is supplied to the input terminal 1, amplified by the BJT amplifier 14a, and then amplified by the BJT amplifier 14b. The signal is output from the output terminal 2. The amplified RF signal output from the output terminal 2 is allowed to pass through the input terminal (RF IN) 3 for receiving an RF signal, and then supplied to the mixer 15. Since the local signal has been supplied to the mixer 15 through the input terminal 4 (LO IN) for receiving the local signal, an intermediate-frequency signal having a differential frequency component between the two signals is generated in the mixer 15, and then output from the output terminal (IF OUT) 5 for outputting the intermediate-frequency signal.
When all of the plural stage amplifiers are the same type BJT amplifiers 14a and 14b, distortions, such as intermodulation distortions, become excessive though noise factors (NF) and gains can be improved. As a result, there arises a problem in that the foregoing structure is not suitable for use in, for example, a mobile communication receiving system which requires a low distortion characteristic.
Although a field effect transistor (hereinafter called as an "FET") of a MOS type or the like manufactured by the same silicon process has an excellent distortion characteristic, it involves a high noise factor and a low gain. Therefore, the foregoing transistor is not suitable for use as the low-noise amplifier as compared with the BJT. When the BJT is considered as a large output amplifier, the BJT having a large gain and involving excessive distortion encounters a problem in that it is not suitable for use as a linear amplifier which must have a low distortion characteristic. Since each MOSFET has a low gain, the number is increased when the same gain is required, thus resulting in a problem to arise in that the electric power consumption is enlarged and thus the size of the circuit is enlarged excessively. Note that the detailed structure of the circuit in the amplifier is disclosed in the cited document.
FIG. 16 shows the detailed structure of a circuit in the mixer 15 disclosed in The Institute of Electronics, Information and Communication Engineers (IEICE) Technical Report, MW 96-72, pp. 27-33, 1996, Department for the Study of Microwave of Electronic Information Communication Society, 1996 and shown in FIG. 15. Referring to the drawing, reference numerals 6a to 6c represent power supply terminals, 31a and 31b represent silicon BJTs and 34a to 34c represent resistors. The two BJTs 31a and 31b are connected in series, that is, the emitter of the silicon BJT 31b is connected to the collector of the other silicon BJT 31a. The RF signal is received by the base of the silicon BJT 31a through the input terminal (RF IN) 3, while the local signal is received by the base of the silicon BJT 31b through the input terminal (LO IN) 4. The intermediate-frequency signal having a different frequency component between the RF signal and the local signal is fetched from the collector of the silicon BJT 31b, and then output through the output terminal (IF OUT) 5. The resistors 34a to 34c are bias resistors for supplying DC power source from the power supply terminals 6a to 6c to the two silicon BJTs 31a and 31b .
Although the BJT has a characteristic in that the noise factor (NF) is low and the gain is high as compared with the MOSFET manufactured by the same process, it has a problem in that it can easily be saturated and has large distortion. If the intermediate-frequency signal or a base band signal is supplied in place of the RF signal, the BJT as well as acts as a transmitting mixer.
Since the BJT amplifiers 14a and 14b and the mixer 15 composed of the two silicon BJTs 31a and 31b can be manufactured by the same silicon semiconductor process, they can be monolithically integrated on one silicon semiconductor chip 20, as shown in FIG. 15. If larger mixer local electric power is required, the local amplifier generally comprises the BJT and is monolithically integrated on the same silicon semiconductor chip 20 having the mixer and the like monolithically integrated thereon.
The conventional RF circuit fabricated as described above such that all of the plural stage amplifiers are the same type transistors however suffers from high distortion though an excellent noise factor (NF) can be realized if the RF circuit comprises the same type BJTs as the low-noise amplifier. Therefore, there arises a problem in that the foregoing RF circuit is not suitable for use as a receiving system which must satisfy both of the low noise characteristic and the small distortion characteristic. If a large output amplifier comprising the BJT is fabricated, the amplifier of the foregoing type is not suitable for use as a linear amplifier which must have a low distortion characteristic because of the large distortion though a large gain can be realized.
Although an RF circuit of the type composed of a mixer comprising only the BJTs is able to obtain a large conversion gain, there arises a problem in that the RF circuit of this type is not suitable for use as a receiving mixer which must have a low distortion characteristic because of too large distortion. Another problem arises in that it is not suitable for use as a transmitting mixer.
An RF circuit of the type in which the amplifier and the mixer are monolithically integrated on one chip has the structure such that all of the RF semiconductor devices are the BJTs. Therefore, there arises a problem in that the RF circuit of the foregoing type is not suitable for use as an RF circuit which must satisfy both of the low noise characteristic and a small distortion characteristic.
Although the mixer having the structure in which the BJT is employed as the local amplifier is able to operate with small local electric power, it suffers from a problem in that the noise of the mixer is intensified because the local amplifier can easily be saturated and noise is added to the local signal.